1. Field of the Invention
The present invention relates to an image forming method and to an image forming apparatus, such as copiers, printers and facsimile machines, using the image forming method, and more particularly to an image fixing method of color toner images including a color toner and a black toner and to an image forming apparatus using the image fixing method.
2. Discussion of the Background
Various electrophotographic full color image forming methods using three color toners of yellow, magenta and cyan toners, or four color toners of yellow, magenta, cyan and black toners have been proposed and practically used.
When the surface of color toner images is smooth, the toner images have high gloss because the surface of the toner images reflects light like a mirror. To the contrary, when toner images have rough surface, the toner images look mat because light randomly reflects at the surface of the toner images. Therefore, when full color toner images are fixed by application of heat, the color toner images are typically fixed at a relatively high temperature to decrease the melt viscosity of the color toners constituting the color images, i.e., to prepare toner images having high gloss.
In addition, when full color images are fixed such that the melted toner images have a low melt viscosity, part of the toner images tends to adhere to the fixing element when the toner images are separated from the fixing element. Therefore the toner images tend to be broken at an inner portion of the toner image layers, i.e., the toner image layers are forcibly separated into two layers (hereinafter sometimes referred to as interlaminar separation) and an offset problem occurs in that all or part of the toner images adhered to the fixing element is re-transferred to the other area of the copy sheet or the next copy sheet. Therefore an oil is typically supplied to fixing elements to avoid such an offset problem.
In attempting to solve the offset problem, Japanese Patent Publication (hereinafter JPP) No. 51-29825 discloses an electrophotographic fixing method, and Japanese Laid-Open Patent Publications (hereinafter JOPs) Nos. 63-118291, 63-118292 and 63-118293 disclose thermal transfer methods.
In the fixing method of JPP 51-29825, a toner image to be fixed is heated at a fixing device using a film sheet, and then the toner image is cooled while the toner image adheres to the film sheet. The toner image is separated from the film sheet after the toner image is solidified. In addition, JPP 51-29825 discloses air blasting and water cooling to forcibly cool the toner image.
JOPs 63-118291, 63-118292 and 63-118293 disclose hot melt recording media which include a wax ink having a low melt viscosity and which do not cause the offset problem even when images are continuously recorded. Hot melt recording media typically include a wax in the ink as a main component. Although the melt viscosity of waxes is from 10 to 104 centi-poise, the offset problem hardly occur.
Specifically, according to the technique, the offset problem can be avoided if the wax ink is melted and then cooled below a temperature at which the wax ink has a top peak value when analyzed by a differential scanning calorimeter (i.e., DSC). In particular, JOP 63-118291 discloses accelerative cooling methods using air blasting or water or freon gas. JOP 63-118292 discloses a cooling method in which the receiving material having an ink image thereon is cooled by a cooling device while the receiving material is contacted with a film sheet. JOP 63-118293 discloses a mechanism which separates the film sheet and the receiving material, and another mechanism which maintains the contact of the receiving material with the film sheet until they are separated.
According to these methods, the offset problem in that (toner) images on a receiving material adhere to a film sheet (i.e., a fixing element) can be avoided even when the images have low melt viscosity.
However, the gloss of the color images prepared by such image fixing methods is not satisfactory although the gloss is higher than general monochrome images. To the contrary, the gloss of the black toner image prepared by such image fixing methods is too high, and therefore the black image is hard to read. Therefore images having both the black and color images have low visual qualities.
In addition, when color images are fixed with a fixing roller at a temperature at which the color toners constituting the color images have low melt viscosity, an offset problem in which the color toners adhere to the fixing roller tends to occur. In addition, a release agent such as oils, is typically applied to the fixing roller to avoid the offset problem. Therefore, a release agent applying device is needed, and other problems occur, such that the image forming apparatus becomes large in size and manufacturing costs of the image forming apparatus increase.
Further, when the fixing method in which the fixing temperature is increased is used, the power consumption of the image forming apparatus increases. Therefore a requirement of low power consumption (i.e., a requirement to save energy) cannot be satisfied.
The gloss of images is often determined depending on individual preference.
Because of these reasons, a need exists for an image forming method and apparatus by which glossy color toner images (including a black image) or color toner images having glossy color images and a mat black image can be selectively obtained without causing the offset problem, without a lubricant applicator (i.e., without increasing manufacturing cost of the image forming apparatus) and without complex operations.
Accordingly, an object of the present invention is to provide an image forming method and apparatus by which glossy color toner images including a glossy black image or color toner images having glossy color images and a mat black image can be selectively obtained without causing the offset problem, without increasing manufacturing cost of the image forming apparatus and without complex operations.
Briefly this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image forming apparatus including:
a fixing unit configured to fix a toner image on a support having at least one color toner and a black toner upon application of heat and pressure thereto, wherein the fixing unit includes:
a fixer configured to heat the toner image to a first temperature not lower than a temperature at which the at least one color toner and the black toner at least soften and then cool the toner image to a second temperature lower than a temperature at which the at least one color toner and the black toner solidify while the surface of the fixer keeps contact with the toner image; and
a pressure applicator configured to press the toner image toward the fixer upon application of pressure P at least when the toner image is heated at the first temperature; and
wherein the surface of the fixer has a ten-point mean roughness Rz not greater than 20 xcexcm, and wherein the pressure P satisfies the following relationships:
((1/100)xc3x97log xcex7c)/15xe2x89xa6Pxe2x89xa6((9/5)xc3x97log xcex7cxe2x88x924/5)/15, and 
((1/100)xc3x97log xcex7b)/15xe2x89xa6Pxe2x89xa6((9/5)xc3x97log xcex7bxe2x88x924/5)/15, 
wherein P represents the pressure in units of 9.8xc3x97100 N/m, and xcex7c and xcex7b represent viscosities of the color toner and the black toner at the first temperature in units of centi-poise, respectively.
The viscosity xcex7c of the color toner is preferably from 10 to 1013 centi-poise. The viscosity xcex7b of the color toner is preferably from 107 to 1018 centi-poise when forming a mat black image, and is preferably from 10 to 1013 centi-poise when forming a glossy black image.
In addition, the ratio xcex7b/xcex7c is greater than 100 when forming a mat black image, and is not greater than 100 when forming a glossy black image.
The image forming apparatus preferably has a cooler configured to actively cool the toner image.
In addition, the image forming apparatus preferably has a fixing temperature controller configured to change the fixing temperature such that a combination of glossy color images and a mat black image is produced or a combination of glossy color images and a glossy black image is produced. The image forming apparatus may have a switch to choice the combination of glossy color images and a mat black image or the combination of glossy color images and a glossy black image. Alternatively, the image forming apparatus may include a printer driver which is connected with a computer and which is configured to control the fixing unit so as to produce the combination of glossy color images and a mat black image or the combination of glossy color images and a glossy black image.
In another aspect of the present invention, an image forming method including the steps of:
forming a toner image on a support, which comprises at least one color toner and a black toner each having a melting point or a softening point;
heating the toner image on the support with a fixer upon application of pressure P while the toner image contacts a surface of the fixer such that the toner image is heated at a fixing temperature not lower than the melting points or softening points of the color toner and the black toner;
then cooling the toner image to a temperature lower than the melting points or softening points while the toner image keeps contact with the surface of the fixer; and
then separating the toner image on the support from the fixer,
wherein the surface of the fixer has a ten-point mean roughness Rz not greater than 20 xcexcm, and wherein the following relationship is satisfied:
((1/100)xc3x97log xcex7c)/15xe2x89xa6Pxe2x89xa6((9/5)xc3x97log xcex7cxe2x88x924/5)/15, and 
((1/100)xc3x97log xcex7b)/15xe2x89xa6Pxe2x89xa6((9/5)xc3x97log xcex7bxe2x88x924/5)/15, 
wherein P represents the fixing pressure in units of 9.8xc3x97100 N/m, xcex7c and xcex7c represent the viscosity of each of the color toner and the black toner in units of centi-poise at the fixing temperature.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.